Quote:
Originally Posted by chillsworld
Would a person need to understand the relationship between the body of the car and smaller object, say a mirror, before doing calculations? That might be poorly worded... Can one treat the smaller object as a stand alone unit when considering boundary layer manipulation, or does one also have to consider the larger objects (Car) impact on the smaller objects boundary layer? Or does it really matter either way?
~C
|
*This is the best question!
*It's only been recently that carmakers have had to consider these small points,driven by the upcoming CAFE standards of 2025.
*And we have no current scientific (a Priori ) knowledge of these interactions.
--------------------------------------------------------------------------
*The CFD/Small-scale wind tunnel studies on mirrors which have been shared so far are flawed,in the sense that,when completed,the investigators concluded that only full-scale wind tunnel testing or Direct Numerical Simulation would yield accurate results.
--------------------------------------------------------------------------
*Earlier testing in the late fifties was on Cd 0.55 cars with so much turbulence,that a small side mirrors drag would only be 1% of the overall drag.
*Cd 0.30 cars are reported to see 2-7% drag associated with mirrors.
*The mirrors on the 'Spirit of Ecomodder.com' added 10% drag.
*The Cd 0.154 Ford Probe-IV was one of the last concept cars which actually had side mirrors.
--------------------------------------------------------------------------
*As to the mirrors,I would analyze them as a small structure in free flight,then add in some interference drag component as a start,when integrated onto the car.
*We don't yet know what the interactions are for sure.And they'll be on a case-specific-basis.
*But there are drag coefficients available for just about any 'components' we might choose to create small structures.
*Local airflow will be a supervelocity compared to the road speed and needs to be guestimated.